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Laser Instant Switch ( sensor )

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goldperson2005

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Hi guys
I need Laser Instant Switch for my project . I've searched about it trough the web , but the laser switching time were about ms(fall and rise time ) , and in phototransistors it was about us .
In my project rise/fall time shouldn't be more than 499ps .
I want to use laser to measure the time between two events . The time that I try to measure is about 3.5ns to 1ms ( the resolution should be at least 0.5ns ) .
The response time (both Rise/Fall) of the laser instant switch should be at least 499ps ( ; 499ps after receiving the spectrums of laser it should trigger starting time measurement ) .
Please let me know how should I figure it out ?

- - - Updated - - -----------------------------------------------------------------------------------------------------------------------------------------------------------------

Can any one present LaserDiodes in ps response time ( both Rise/Fall ) ?
 

kripacharya

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Why don't you provide more details about your setup - then maybe someone can suggest an alternate method of how to achieve your end objective.
 

goldperson2005

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Thanks
What kind of detail should I provide ?
I want to use LaserDiode to trigger the start and stop of time measurement .
 

keith1200rs

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It is not clear what you are trying to do. Using a laser to measure the time between two events implies breaking the beam but you talk about switching the laser with a rise time of less that 499ps - a very precise figure. Are you making a time of flight rangefinder? How are you planning to time things - your timing requirement implies a clock of at least 2GHz unless you are using a TDC chip?

Also, a laser is only a light source - what is the detector?

Keith
 

goldperson2005

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thanks
as you said , " Using a laser to measure the time between two events implies breaking the beam " ; Start by breaking the beam & stop by receiving it again .
it's repetitive & should be accurate at least in 0.5ns . If the response time be more than 499ps it may lose . Yes I want to use TDC chip .
I want to use optical fiber and LeaserDiode as detector .
 

keith1200rs

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I want to use optical fiber and LeaserDiode as detector .

A laser diode isn't a detector. I would suggest you look at a fast PIN photodiode for the detector. Assuming you have lots of signal you should be able to use a simple resistive load (50 ohms or less) for the photodiode. However, you will not find it easy to get <499ps rise time. You need to be looking at 0.2mm diameter PIN photodiodes and similarly high speed (1GHz) circuitry. TDCs don't usually require such fast rise/fall times. Acam resolves down to 22ps and specifies a maximum of 200ns rise time although I think for repeatable results I would like a faster rise time than that!

Keith
 

goldperson2005

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Thanks Keith
TDC 2.jpg
They defined just the max range of rise/fall time . I haven't read the whole of the datasheet , but I think it's trying to say that the maximum rise/fall time of input shouldn't be over than 200ns .
but the other point that I can not figure it out :
TDC 1.png
according to the image , it won't measure whole the time .
what is your idea ? does it measure whole the time ?
Is the time measured by TDC different from the time measured with High-Speed Unit of TDC ?
 

keith1200rs

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As far as I understand it in mode 2 you still get the whole time but it measures it with a coarse time and fine time in order to extend the measuring time to 4ms whereas mode 1 will only measure to 2.5us. Mode 2 would normally be used for "slower" measurements such as ultrasound whereas mode 1 would be for optical measurements. I assume you are looking at something like the TDC-GP22.

Keith.
 

goldperson2005

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Yes , I'm looking at TDC-GP22 .
I can not understand this :
" In mode 2 the high-speed unit of the TDC does not measure the whole time interval but only time intervals from START and STOP to the next rising edge of the reference clock (fine-counts) "
according to this part it shouldn't measure whole the time ?
Please let me know how can I be sure that in mode 2 , it measures whole the time ?
 

keith1200rs

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Yes, it measures the whole time. You must also read the sentence that follows: "In between the fine-counts the TDC counts the number of periods of the reference clock (coarse-count)." So, it adds the two together to get the total time (allowing for the difference in resolution between the two counts).

Keith
 

goldperson2005

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Then accuracy strongly should depends on frequency stability ( ; vs all features ) .
 

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